Reduction of noise



March. 28, 1939.

vC. M. BURRILL REDUCTION 0F NOISE Filed AprilV 25, 1956 4 Sheets-Sheet l HQI (Ittomeg March 28,1939. .QMBURRILL 1 2,151,739

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Ilan-sns Zmventor Chales M. Burril L Gttorneg Patented Mar. 28, 1939 REDUCTION F NOISE Charles M. lurrill, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April l25,

7 Claims.

This invention relates to the reduction of noise such as that likely to be produced in radio receivers and like apparatus by electrical impulses extraneous to the signal to be received, and has for its 4principal object the provision of an improved apparatus and'method of operation whereby the wave shape of the noise producingimpulse is so modiiied as to minimize its disturbingeffect on the signal deliveredl at the receiver output.

A further object of the invention is the provision of an improved noise suppressor circuit whichk (l) voperates with a predetermined relatively short time delay, (2) is operated from a control point preceded byga channel section oi' relatively broad selectivity and followed by a channel section .of relatively sharp selectivity, (3) is of the forward feed type, (4) operates in response to impulses of a predetermined amplitude to reduce or limit the channel response to the signal, and (5) is adapted for use in combination with a vbackward feed automatic volume control system of the usual type.

Previous methods of noise reduction have been directed toward elimination of the disturbance by balancing, or to the reduction of the energy of the disturbance by means of selective circuits, directive antennas or limiters. These methods, however, cannot be vcompletely successful With random interference from random directions or sources When the amplitude of the interference is substantially greater than that of the desired signal. It is to theamelioration of such .cases that this invention is directed. Instead of attempting the hopeless task of eliminating such interference, I seek to alter its character in such a Way as to minimize its ultimate disturbing effect.

For example, if the ultimate purpose of the communication is the hearing of speech or' music 40 for entertainment, I seek to make the interference least disturbing to the pleasure of the reception.

If the purpose of the communication is ythe conveying of intelligible speech, I seek to make the interference such as to be least disturbing to the 45 intelligibility of the reception. The Adesired results are obtained in the two cases just cited by y applying the general principle which I have discovered, namely, that extraneous lowV frequency components are more deleterious to the entertainment value vof ymusic, whilst extraneous high frequencycomponents are most harmful to the intelligibility of speech. Accordingly, I use rapid acting control circuits, sensitive to the onset of sginterference of greater amplitude than that of 1936, Serial No. 76,363

(Cl. l250m-2,0)

the desired signal, to alter the wave form of the disturbance in the desired manner.

Many other applications of these principles may be made, and various specific' circuit arrangements may be used suitable to each application. By way of illustration, I shall describe the application of the invention to a radio broadcast receiver to be used for entertainment.

The invention will be better understood from the following description when considered in connection with the accompanying drawings and its scope is indicated by the appended claims.

Referring to the drawings:

Figures 1 to 9 are explanatory diagrams relating to the operation of the invention, and

Fig. l0 is a Wiring diagram illustrating the application of the invention to one type of radio receiver. Y

Fig. 1 shows the alternating voltage at the grid of the second I. F. tube in a radio receiver, the point at which the control voltage is derived, when an unmodulated carrier is being received. This voltage is held approximately constant, Whatever the magnitude of the impressed carrier, by means of a slow yacting automatic volume control circuit. By slow I mean slow compared with the lowest modulating frequency ,to be received. This automatic volume control circuit is of the amplified type, in which the direct current produced in the second detector is amplified by the A. V. C. tube before being fed Vback to control the gain of the radio frequency, first detector and first intermediate frequency tubes. This type of automatic volume control is very desirable When a noise reduction control circuit is `to be used, for it can be made to hold the required voltage constant to a much closer degree of approximation than is possible with other circuits more commonly used. This makes possible a more accurate fixed adjustment of the threshold or delay in the action vof the noise reduction control circuit, and so improves the functioning of this circuit.

Fig. 2 corresponds to Fig. l except that the received carrier is sinusoidally modulated 75% at about the highest modulation frequency permitted by the overall selectivity of the receiver, including `the audio frequency amplier and loudspeaker, in other Words, the highest useful modulation frequency. This may be of the order of 5K0. Figs. 1 and 2 have been included so that the igures to follow, which will be drawn to the same scale, may be studied with Vdue consideration for relative magnitudes.

Fig. 3 shows the damped Wave train or oscillatory pulse produced at the grid of the second intermediate frequency tube, in the absence of any received carrier, when an impulsive disturbance of very short duration is impressed on the input of the receiver, for example, a single pulse of automobile ignition interference. The shape shown for the ensuing damped oscillation is to be taken as illustrative only, for it will depend very greatly on the transient characteristics of the particular receiver. onant circuits in the receiver are properly lined up, it will in general be symmetrical about the zero axis. Furthermore, its total duration will be of the same order of magnitude as the duration of a half cycle of the highest modulation frequency which would be permitted by the selectivity of the receiver preceding the point in question (a frequency equal to half the amplifier band width). The frequency of this noise pulse will be approximately the central or resonant frequency of the intermediate frequency amplifier, but in general, for a multistage amplifier it will not be constant.

It is desirable for noise reduction 'tol have as much of the total selectivity of the receiver as is practical following the point at which the noise `reduction control voltage is derived. This point we shall hereafter call the control point. The reason for thus apportioning the selectivity will be clear from the explanation to follow. For this reason the noise pulse in Fig. 3 is shown substantially shorter in duration than a half cycle of the modulation shown in Fig. 2, corresponding to less selectivity at the control point than at the output of the receiver.

Fig. 4 shows a combination of a modulated carrier, as shown in Fig. 2 and a noise pulse as shown in Fig. 3. In deriving this figure it has been assumed for simplicity in explanation that the oscillation comprising the noise pulse is of constant frequency equal to that of the carrier, and that it is in phase with the carrier. In general this will not be the case, and beats will be produced between the noise pulse oscillation and the carrier, making the envelope of the resultant more irregular than that shown. However, the period of such beats will be of the order of or greater than the duration of the entire pulse, so that in any case the envelope of the voltage will not be greatly different from that shown in Fig. 4.

The horizontal line A in Fig. 4 indicates the threshold level at which the noise reduction control circuit begins to function. In other words, the noise reduction control circuit is delayed by a voltage OA. This is obtained by adjusting the bias on the noise rectifier, produced by the voltage drop racross resistor R2 (Fig. 10) until it equals the voltage OA times the voltage amplification of the noise control amplifier stage. This adjustment may be accomplished by varying the resistance of R2 (Fig. l0) or in any other convenient manner. 'Ihe threshold level or delay shown corresponds to 150% modulation of the received carrier, selected as a value likely to be practical in operation. For the greatest noise reduction, it should be set as low as possible without having the noise reduction control circuit actuated by the desired signal. Evidently how `low it can be set depends on the constancy of operation of the receiver, the effectiveness of the automatic volume control, and the accuracy of all adjustments. If the threshold is established too low, the noise reduction control circuit will be actuated by the peaks ofthe desired However, when the resmodulation, and consequently these peaks will not be properly reproduced. However, in cases of severe interference it may be found preferable on the whole to set the threshold at as low as '70% modulation, in order to obtain greater noise reduction even at the expense of some distortion of the modulation peaks.

Fig. 5 shows the control voltage produced by the wave form shown in Fig. 4 across resistor R1 (Fig. 10) by the action of the noise control amplifier and rectier. The horizontal line represents the control voltage required to completely cut off the tube controlled, in the present case, the second intermediate frequency tube. In drawing this figure the time delay and broadening due to the selectivity of the noise control amplifier stage has been neglected. This is justified 1f this stage is very broad, as should be the case. The speed of the noise reduction control is thus determined principally by the noise rectifier circuit. Such circuits are in general characterized by two time constants, a build-up time and a die away time. These time constants are determined by the various resistances and'capacitances in the circuit, including those of the rectifier itself. Usually these time constants are not equal, and usually the die-away time is greater, as shown in Fig. 5. However, many other well known rectier connections may be used to give other desired time constants. Besides these time constants, Fig. 5 also indicates two time intervals of interest, the rst, which may be called the action time, between the beginning of the noise pulse and complete cutoi by the control circuit; and the second, which may be called the release time, between the end of the noise pulse and the complete release -by the control circuit. In the illustration of Fig. 5 the release time is negative, that is, the control is released before the end of the noise pulse.

In Fig. 6 is shown the control characteristic of the tube controlled, in the apparatus of Fig. 10 the second intermediate frequency tube, controlled by the vbias on its control grid. Many other methods of control may be used, for example, the voltage on the suppressor or screen grid may be varied if a pentode tube is used. Or a tube with an auxiliary control electrode, such as the RCA Radiotron 6L'7 may be used. In fact, the control may be applied to any tube electrode or simultaneously to a plurality of electrodes.

Fig. 9 has been constructed from Figs. 5 and 6 to show the variation with time, of the amplification of the tube controlled, amplification being taken as that obtained when the noise reduction control circuit is not energized. IfA the noise reduction control circuit were not energized, the amplification would remain constant at 100% and the waveshape of the plate current of the controlled tube would be the same as that of the grid voltage, shown in Fig. 4; for the pentode tube used may be regarded to a sufficiently accurate approximation as a generator of a constant current proportional toits grid voltage. But with the noise reduction control circuit in operation, the amplification varies as shown in Fig. 9, producing a plate current wave form such as is shown in Fig. 8.

The waveform at the output of the audio frequency amplifier is related to that of the plate current of the second intermediate frequency tube shown in Fig. 8 but is modied by the detection and by the smoothing action of selective circuits traversed, both at intermediate and ataudio frequency. Thus the final output waveform may appear as in Fig. 9. There are also indicated by dotted lines in this figure, for cornparison, the waveform which would have resulted had there been no noise, and the waveform which would have been produced with the noise present and reduced by the control circuit, if

the amplifier following the control point had been very broad (nonselective). The advantageous action of selectivity following the control point in smoothing out the noise disturbance is evident.

In Fig. 7, both waveforms applying to cases when noise is present contain D. C. components. That is, the area under positive portions of the wave is not equal to the area under the negative portions. Since in general the audio amplier will not be responsive to D. C. components, such components will initiate a low frequency transient in the audio amplifier, of long duration and disturbing audibility. Hence, it is desirable to so choose the time constants of the noise reduction control circuit, taken in conncction with the selectivity characteristics of the receiver, that such D. C. components are reduced to as small an amplitude as possible, on the average. This can be done by an analysis similar to that here carried out in Figs. 1 to 9, or by experiment using a cathode ray oscillograph or other means of waveform examination.

As indicated by Fig 10, one suitable embodiment of the invention is a radioreceiver including an antenna l5 from which signal impulses vare supplied to a loudspeaker l 6 through a channel including a radio frequency stage I1, a first detector i8, intermediate frequency stages I9 and 2u, a second detector 2|, tone control device 22, volume control device 23, a push-pull driver stage 24 and a push-pull output stage 25. An automatic volume control tube 26 is subjected to a grid control potential derived from the output circuit of the second detector 2l and operates through its output circuit to regulate the gain of the leading stages to the channel. These parts of the apparatus are well known and their operation will be readily understood without detailed explanation.

The present invention involves the addition, to these wellV known features, of a noise control or suppressor tube 2 which is subjected toan input grid or control potential applied from the input circuit of the second intermediate frequency tube 2li through a capacitor 28 and a resistor 29. It will be noted that the tube 2l is of the diode detector amplier type; that the amplier output is supplied through a transformer 3f! to the diode detector circuit which includes` a resistor R1 shunted by a condenser C1 and a resistor R2 shunted by a condenser C2; and that the resistor R1 and condenser C1 are interposed in the grid or control circuit of the second intermediate frequency tube 20. 'I'his noise reduction control circuit is similar to Well known automatic volume control circuits but differs therefrom in the following respects:

First, it is rapid acting but not instantaneous. By this I mean that the time necessary for it to respond is small but not negligible. If such speed were used for automatic volume control the low frequency modulation components would be removed from the desired signal. However, such an undesired result in an auto-matic volume control is the very result sought in a noise reduction control circuit. This speed o-f action is determined largely by the value of R1, and the capacitance across it, either intentionally-introduced by the capacitor C1 or simply that inevitably due to the wiring.

Second, the action of the noise reducing control circuit is delayed so as to operate only on peaks of interference exceeding the desired signal in amplitude. This is accomplished by means of a constant bias on the noise rectier comprising the drop in potential across resistor R2 produced by the plate current of the amplier tube preceding the noise rectifier. Other well known methods of obtaining the desired delay or of biasing the rectifier may be used.

Third, the control voltage is fed forward, instead of backward as is usual in automatic volume control circuits. That is, the control is applied beyond the point where the voltage actuating the contro-l is derived. By so doing, the operation of the control to reduce the final output does not change significantly the voltage actuating the control. In the embodiment shown, the voltage impressed on the grid of the second intermediate frequency amplifier tube is also used to actuate the noise reducing control circuit. The control voltage derived is then used to vary the gain of this second intermediate frequency amplifier tube, without in so doing affecting api-'1 preciably the voltage impressed on its grid, also used for actuating the control. A feed forward circuit is desired in a noise reducing control circuit, because such a control, while actuated, tends to reduce the output to zero and keep it there, and actually produces such a result unless prevented by overloading or saturation in the control circuit. On the contrary, a feed backward circuit can not reduce the output to zero, but instead tends to maintain the output constant. In this respect, such a circuit used for noise reduction is the practical equivalent of a limiter. I have found feed forward control circuits definitely .superior to feed backward control circuits for noise reduction, because of the greater variety of response characteristics available for producing the desired alteration of theV noise waveshape to minimize its ultimate disturbing eifect.

I realize that the simple use of a feed forward control circuit is proposed in U. S. Patent No. 1,574,780 of Affel and the use of delay bias is proposed in British Patent No. 185,133 of Scott- Taggart. But apparently no one heretofore has recognized in the use of these features for noise reduction, the importance of the various possible transient response characteristics of such circuits, or provided means for utilizing and Vcontrolling these various transient response characteristics for noise reduction, in order to minimize the disturbing character of the interference. The actual characteristic most desirable in any given case will depend on the character and purpose of the desired signal, on the nature of the interference and perhaps on certain circumstances of reception, such for example as the acoustic characteristics of the listening room.

It is apparent that, by varying the speed of action of the noise reduction control circuit, and by varying the selectivity before and after the control point, a great variety of output wave forms may be obtained, as illustrated in a single case by the foregoing figures. In no case will a disturbance greater in amplitude than the signal be entirely eliminated, but in any particular case an adjustment of these various characteristics of the receiver and noise reduction control circuit may be obtained such as to minimize the disturbing effects of such interference.

I claim as my invention:

l. A signal channel including relatively broad and relatively sharp selectivity channel sections, an amplier provided with a signal input circuit connected to said broad selectivity section and with an output circuit connected to said sharp selectivity section, a noise suppressor device provided wth an input circuit energized in. accordance with the potential of said amplier input circuit and with an output circuit, and an impedance device common to said amplifier signal input and said suppressor output circuits.

2. A signal channel including relatively broad and relatively sharp selectivity channel sections, an amplifier provided with a signal input circuit connected to said broad selectivity section and with an output circuit connected to said sharp selectivity section, a noise suppressor device provided with an input circuit energized in accordance with the potential of said amplifier input circuit and with an output circuit, an impedance device common to said amplifier signal input and said suppressor output circuits, and means assdciated with said impedance device for timing the action of said suppressor device.

3. A signal channel including relatively broad and relatively sharp selectivity channel sections, an amplifier provided with a signal input circuit connected to said broad selectivity section and with an output circuit connected to said sharp selectivity section, a noise suppressor device provided with an input circuit energized in accordance With the potential of said amplier input circuit and with an output circuit, an impedance device common to said amplifier signal input and said suppressor output circuits, and means including a capacitor connected in shunt to said impedance device for timing the action of said suppressor device.

Il. A signal channel including relatively broad and relatively sharp selectivity channel sections, an amplifier provided with a signal input circuit connected to said broad selectivity section and with an output circuit connected to said sharp selectivity section, a noise suppressor device provided with an input circuit energized in accordance with the potential of said amplifier input circuit and with an output circuit, an impedance device common to said amplifier signal input and said suppressor output circuits, and means including an impedance device connected in said output circuit for predetermining the impulse amplitude at which operation of said suppressor device is initiated.

5. The combination of a signal channel including a radio frequency amplifier, a first detector, a plurality of intermediate frequency ampliers, a second detector and an audio frequency amplifier, a relatively slow circuit energized from the input of said second detector for controlling the gain of said radio frequency amplifier and the first of said intermediate frequency amplifiers, and a relatively fast noise suppressor circuit provided with an input connection energized from the signal input circuit of the second of said intermediate frequency ampliers and with an output connection, means providing relatively broad selectivity preceding and relatively sharp selectivity following said second intermediate frequency amplifier, and means including an impedance device common to said output connection and said second intermediate frequency amplifier signal input circuit for timing the action of said suppressor circuit.

6. A noise suppressor for a radio receiving system operative to alter the Wave form rapidly above a predetermined amplitude and comprising in combination, means providing a signal amplifying channel, means providing a control point for said noise suppressor on said channel, means providing in said channel a higher degree of selectivity following said point than preceding said point, a noise suppressor tube coupled to said control point to receive the signals and noise impulses therefrom, means for rectifying said noise impulses, means for adjusting said rectifying means to respond to noise impulses above a predetermined amplitude, an output resistor for said rectifying means, and means providing a bias control connection therefrom to control the gain of the channel in a forward direction from said control point.

'7. A noise suppressor for a radio receiving system operative to alter the wave form rapidly above a predetermined amplitude and comprising in combination, means providing a signal amplifying channel, means providing a control point for said noise suppressor on said, channel, means providing in said channel a higher degree of selectivity following said point than preceding said point, a noise suppressor tube coupled to said control point to receive the signals and noise impulses therefrom, means for rectifying said noise impulses, means for adjusting said rectifying 4means to respond to noise impulses above a predetermined amplitude, an output resistor for said rectifying means, means providing a bias control connection therefrom to control the gain of the channel in a forward direction from said control point, means for deriving an automatic volume control potential from said channel following said control point, and means for applying said last named potential to said channel to control the gain thereof preceding said control point.

CHARLES M. BURRILL. 

